1. Field of the Invention
The present invention generally relates to a process for forming carboxylic acids; more particularly, to a rhodium-catalyzed one-step process for forming a carboxylic acid directly from the corresponding olefin and to a rhodium-catalyzed process for oxidizing an aldehyde to form the corresponding carboxylic acid.
2. Description of the Prior Art
Carboxylic acids have many uses in the chemical industry. For example, propionic acid is useful as a grain preservative and higher acids have been used in the manufacture of detergents.
Many different processes are known for preparing carboxylic acids, such as hydroformylation of an olefin followed by oxidation of the resulting aldehyde. The acid produced has a carbon number one higher than the olefin. For example, an olefin (e.g., ethylene) is hydroformylated by reaction with carbon monoxide and hydrogen to produce the corresponding aldehyde (propionaldehyde) which is then in turn oxidized to the corresponding acid (propionic acid). In some cases, after the hydroformylation reaction of the olefin with carbon monoxide and hydrogen, also known as an oxo process, the aldehyde is recovered and purified before oxidizing it to the acid.
Other related processes of the prior art do not require purification of the aldehyde. For example, West German Pat. No. 2,604,545 discloses a two-step method for preparing alkylcarboxylic acids by hydroformylating the corresponding olefin at high pressures of 100 to 600 bars and directly oxidizing the resulting aldehyde-containing reaction mixture, both steps being conducted in the presence of a rhodium carbonyl complex catalyst.
U.S. Pat. No. 3,980,670 discloses a process for simultaneously obtaining both methacrylic acid and butyrolactone by first hydroformylating allyl esters of lower fatty acids in the presence of both rhodium carbonyl complex catalysts and inert organic solvents, followed by directly oxidizing the resulting reaction mixture in the presence of lower fatty acids and recovering the products after separation of a residue including the rhodium catalyst.
U.S. Pat. No. 3,520,937 discloses a technique for processing cobalt- and aldehyde-containing oxo reaction mixtures with an oxidizing agent to provide a cobalt-free material. The amount of oxygen is disclosed to be insufficient to oxidize the product aldehyde.
Still other prior art processes provide for directly oxidizing or carboxylating an olefin to produce various products. For example, U.S. Pat. No. 3,384,669 discloses a process for converting an olefin to the corresponding aldehyde or ketone by oxidation with molecular oxygen in the presence of a catalyst which comprises an aqueous solution of varivalent noble metal ions and either nitrate or nitrite ions or a mixture thereof.
West German Pat. No. 2,744,207 discloses a method of oxidizing an olefin to, for example, a ketone by reacting the olefin, in the absence of an inert solvent to facilitate product recovery, in a reaction medium comprising an organic phosphine or phosphite in the presence of a rhodium complex catalyst and a stabilizing phosphine or phosphite ligand.
U.S. Pat. No. 3,818,060 discloses a Group VIII metal-catalyzed hydrocarboxylation process for forming carboxylic acids comprising reacting an olefin with carbon monoxide and water. The catalyst system comprises an iridium or rhodium-containing compound, a halide promoter and a stabilizer composed of an organic derivative of pentavalent phosphorus, arsenic, antimony, nitrogen or bismuth. The stabilizers are disclosed as preventing precipitation and solids deposition which would ordinarily adversely affect catalyst stability. See also U.S. Pat. Nos. 3,816,488, 3,816,489 and 3,944,604.
Blum et al., in Tetrahedron Letters No. 38, pp. 3665-3668 (1967) disclose the alpha-oxidation of alkylbenzenes by the rhodium complex catalyst chlorotris(triphenylphosphine) rhodium, RhCl(PPh.sub.3).sub.3 (where "Ph"=phenyl). Specifically, the alkylbenzene ethylbenzene is oxidized by air to acetophenone in the presence of the catalyst.
Fusi et al., in Journal of Organometallic Chemistry 26 (1971) pp. 417-430, propose a mechanism for the oxidation of cyclohexene by transition metal complexes. Specific results are given for the oxidation of cyclohexene to cyclohexene oxide, cyclohexanone and cyclohexanol in the presence of various catalysts, including rhodium complexed with triphenyl phosphine.
Takao et al., in Bulletin of the Chemical Society of Japan, 43 (12) December, 1970, pp. 3898-3900, report on the oxidation of the olefin styrene with the rhodium complex chlorotris(triphenylphosphine) rhodium or rhodium chloride, and the effect of various solvents on the oxidation products. In a non-polar solvent such as toluene, the main products were both acetophenone and benzaldehyde. The oxidation of methylstyrenes with the same catalysts is also reported.
In a later publication by Takao et al., Bulletin of the Chemical Society of Japan, 45(5) May, 1972, pp. 1505-1507, the oxidation of cinnamaldehyde catalyzed by rhodium complexes in various solvents is reported. Two rhodium complexes, chlorocarbonylbis(triphenylphosphine) rhodium and chlorotris (triphenylphosphine) rhodium, were found to cause the catalytic oxidation of cinnamaldehyde in toluene to give benzaldehyde, glyoxal, benzene and styrene.
In Bulletin of the Chemical Society of Japan, 45(7) July, 1972, pp. 2003-2006, Takao et al. report on the oxidation, in a solvent, of vinyl esters catalyzed with chlorotris (triphenylphosphine) rhodium. The particular reaction products obtained are determined, in part, by the substitutent on the olefinic carbon atom. For example, vinyl acetate was oxidized in toluene to give acetone, propionaldehyde and methyl vinyl ether; while methyl ethyl ketone, butyraldehyde and ethyl vinyl ether were obtained from vinyl propionate.
Dudley et al., in J.C.S. Dalton, (1974) pp. 1926-1931, disclose the rhodium-promoted oxidation of .alpha.-olefins to methyl ketones in benzene. Two rhodium complexes, chlorotris(triphenylphosphine) rhodium and carbonylhydridotris (triphenylphosphine) rhodium, are shown to catalyze the reaction.
Mercer et al., in Journal of the American Chemical Society 97 (7) April, 1975, pp. 1967-1968, disclose that the rhodium complex Rh.sub.6 (CO).sub.16 catalyzes the oxidative cleavage of carbon-carbon bonds in ketones to carboxylic acids. A specific reaction reported involved suspending the rhodium in cyclohexanone as a solvent and pressurizing with oxygen, to produce adipic acid.
Various other prior art disclose methods of hydroformylation in which the presence of oxygen retards the reaction. For example, Polievka et al., in Petrochemia 1979, 19 (1-2), 5-12, disclose the low pressure hydroformylation of olefins (1-octene, di- and tri-isobutylene, allyl alcohol, styrene and dipentene) with a rhodium complex catalyst, HRh(CO) (PPh.sub.3).sub.3 (where "Ph"=phenyl). The authors disclose that compounds such as oxygen, which are more reactive toward the catalyst than alkenes or CO, form stable complexes which retard the hydroformylation.
Matsui et al., in Bulletin of the Japan Petroleum Institute, 19, No. 1, May 1977, propose a mechanism to explain the observed deactivation of rhodium complex catalysts used in hydroformylation reactions. The specific catalysts reported on comprise rhodium, as Rh.sub.2 Cl.sub.2 (CO).sub.4, complexed with a triphenyl phosphite ligand. The authors concluded that catalyst deactivation was mainly due to the oxidation of triphenyl phosphite to triphenyl phosphate by the small amount of oxygen present in the synthesis gas.
The prior art also teaches the reactivation of deactivated rhodium complex catalysts with oxygen. For example, Japanese Pat. No. 51-23212 discloses a rhodium-catalyzed hydroformylation process and particularly a technique for reactivating the rhodium catalyst which becomes deactivated during the process by treating the deactivated rhodium catalyst in a separate step with oxygen and then recycling the reactivated catalyst back to the hydroformylation reaction.
Commonly-assigned, copending U.S. patent application Ser. No. 703,130 (published as Belgian Pat. No. 856,542) now U.S. Pat. No. 4,221,743 discloses a hydroformylation process in which a deactivated rhodium complex catalyst may be reactivated by bleeding small catalytic quantities of oxygen into the hydroformylation reaction system. The amount of oxygen employed is small (i.e., sufficient only to detoxify and reactivate the catalyst) and substantially all of the oxygen is consumed by the ligand in the catalyst to free the catalytic rhodium and thereby reactivate the catalyst.
Other prior art disclose hydroformylation processes where oxygen is present. For example, U.S. Pat. No. 3,920,754 discloses a hydroformylation process for forming formyl- and hydroxymethyl-substituted alkene derivatives by reacting the alkene with carbon monoxide and hydrogen in the presence of a free-radical initiator which preferably is molecular oxygen.
U.S. Pat. No. 3,954,877 discloses an olefin hydroformylation process which employs a complex of a Group VIII metal (e.g., rhodium) with a ligand comprising a pentavalent phosphorus, arsenic or antimony compound (e.g., phosphine oxides).
U.S. Pat. No. 3,555,098 discloses a Group VIII noble metal-catalyzed hydroformylation reaction wherein catalytic activity is maintained by treating all or a portion of a recycled reaction medium containing the catalyst with an alkaline aqueous solution to extract by-product carboxylic acids which otherwise deactivate the catalyst. The acids are believed to be formed by the slight oxidation of the product aldehyde due to oxygen contamination of the reactant gas streams.
For many years, all commercial hydroformylation reactions employed cobalt carbonyl catalysts which required relatively high pressures (often on the order of 100 atmospheres or higher) to maintain catalyst stability. U.S. Pat. No. 3,527,809, issued Sept. 8, 1970, to R. L. Pruett and J. A. Smith, discloses a significantly new hydroformylation process whereby alpha-olefins are hydroformylated with carbon monoxide and hydrogen to produce aldehydes in high yields at low temperatures and pressures, where the normal to iso- (or branched-chain) aldehyde isomer ratio of the product aldehydes is high. This process employs certain rhodium complex catalysts and operates under defined reaction conditions to accomplish the olefin hydroformylation. Since this new process operates at significantly lower pressures than required theretofore in the prior art, substantial advantages were realized including lower initial capital investment and lower operating costs. Further, the more desirable straight-chain aldehyde isomer could be produced in high yields.
The hydroformylation process set forth in the Pruett and Smith patent noted above includes the following essential reaction conditions:
(1) A rhodium complex catalyst which is a complex combination of rhodium with carbon monoxide and a triorganophosphorus ligand. The term "complex" means a coordination compound formed by the union of one or more electronically rich molecules or atoms capable of independent existence with one or more electronically poor molecules or atoms, each of which is also capable of independent existence. Triorganophosphorus ligands whose phosphorus atom has one available or unshared pair of electrons are capable of forming a coordinate bond with rhodium.
(2) An alpha-olefin feed of alpha-olefinic compounds characterized by a terminal ethylenic carbon-to-carbon bond such as a vinyl group, CH.sub.2 .dbd.CH--. They may be straight chain or branched chain and may contain groups or substituents which do not essentially interfere with the hydroformylation reaction, and they may also contain more than one ethylenic bond. Propylene is an example of a preferred alpha-olefin.
(3) A triorganophosphorus ligand such as a triarylphosphine. Desirably each organo moiety in the ligand does not exceed 18 carbon atoms. The triarylphosphines are the preferred ligands, an example of which is triphenylphosphine.
(4) A concentration of the triorganophosphorus ligand in the reaction mixture which is sufficient to provide at least two, and preferably at least 5, moles of free ligand per mole of rhodium metal, over and above the ligand complexed with or tied to the rhodium atom.
(5) A temperature of from about 50.degree. to about 145.degree. C., preferably from about 60.degree. to about 125.degree. C.
(6) A total hydrogen and carbon monoxide pressure which is less than 450 pounds per square inch absolute (psia), preferably less than 350 psia.
(7) A maximum partial pressure exerted by carbon monoxide no greater than about 75 percent based on the total pressure of carbon monoxide and hydrogen, preferably less than 50 percent of this total gas pressure.
In commercial hydroformylation-oxidation processes for producing carboxylic acids, the resulting aldehyde is usually recovered and then oxidized to the corresponding acid. Where the oxidation step is conducted without an aldehyde purification step and employing Group VIII metal catalysts, such as a rhodium-based catalyst, severe catalyst stability and activity problems are encountered when the oxidation is conducted in the presence of an inert aromatic or aliphatic solvent. Even without inert solvents, in the hydroformylation-oxidation route of olefin to acid, it has been necessary in the prior art to conduct the process in two stages (i.e., hydroformylation followed by a separate oxidation without aldehyde purification).